Tiny Emus has in-browser emulators for all the classic 8-bit systems, but also ready links for specific games so you don't have to spend ages tracking them down and configuring them.
Creator Andre Weissflog, via Hacker News:
The emulators are written in C(99), with some platform-specific bits written in Objective-C and Javascript, and in C++ for the UI parts (since Dear ImGui is implemented in C++). Apart from the WebAssembly version, Win32, Linux, OSX and iOS is also supported, but you need to compile those yourself
